Specific cell killing can be essential for the successful treatment of a variety of diseases in mammalian subjects. Typical examples of this are in the treatment of malignant diseases such as sarcomas and carcinomas. However the selective elimination of certain cell types can also play a key role in the treatment of many other diseases, especially immunological, hyperplastic and/or other neoplastic diseases.
The most common methods of selective treatment are currently surgery, chemotherapy and external beam irradiation. Targeted endo-radionuclide therapy is, however, a promising and developing area with the potential to deliver highly cytotoxic radiation to unwanted cell types. The most common forms of radiopharmaceutical currently authorised for use in humans employ beta-emitting and/or gamma-emitting radionuclides. There has, however, been a recent surge in interest in the use of alpha-emitting radionuclides in therapy because of their potential for more specific cell killing. One alpha-emitting nuclide in particular, radium-223 (223Ra) has proven remarkably effective, particularly for the treatment of diseases associated with the bone and bone-surface. Additional alpha-emitters are also being actively investigated and one isotope of particular interest is the alpha-emitter thorium-227.
The radiation range of typical alpha emitters in physiological surroundings is generally less than 100 micrometers, the equivalent of only a few cell diameters. This makes these nuclei well suited for the treatment of tumours, including micrometastases, because little of the radiated energy will pass beyond the target cells and thus damage to surrounding healthy tissue might be minimised (see Feinendegen et al., Radiat Res 148:195-201 (1997)). In contrast, a beta particle has a range of 1 mm or more in water (see Wilbur, Antibody Immunocon Radiopharm 4: 85-96 (1991)).
The energy of alpha-particle radiation is high compared to beta particles, gamma rays and X-rays, typically being 5-8 MeV, or 5 to 10 times that of a beta particle and 20 or more times the energy of a gamma ray. Thus, this deposition of a large amount of energy over a very short distance gives α-radiation an exceptionally high linear energy transfer (LET), high relative biological efficacy (RBE) and low oxygen enhancement ratio (OER) compared to gamma and beta radiation (see Hall, “Radiobiology for the radiologist”, Fifth edition, Lippincott Williams & Wilkins, Philadelphia Pa., USA, 2000). These properties explain the exceptional cytotoxicity of alpha emitting radionuclides and also impose stringent demands on the level of purity required where an isotope is to be administered internally. This is especially the case where any contaminants may also be alpha-emitters, and most particularly where long half-life alpha emitters may be present, since these can potentially be retained in the body and cause significant damage over an extended period of time. Whether long or short half-life, however, radiochemical purity should be as high as reasonably feasible and contamination with non-targeted radionuclides should be minimised.
The radioactive decay chain from 227Ac, generates 227Th and then leads to 223Ra and further radioactive isotopes. The first three isotopes in this chain are shown in FIG. 3. The table shows the element, molecular weight (Mw), decay mode (mode) and Half-life (in years (y) or days (d)) for 227Th and the isotopes preceding and following it. Preparation of 227Th can begin from 227Ac, which is itself found only in traces in uranium ores, being part of the natural decay chain originating at 235U. One ton of uranium ore contains about a tenth of a gram of actinium and thus although 227Ac is found naturally, it is more commonly made by the neutron irradiation of 226Ra in a nuclear reactor.
It can be seen in FIG. 3 that 227Ac, with a half-life of over 20 years, is a very dangerous potential contaminant with regard to preparing 227Th from the decay chain for pharmaceutical use. Even once the 227Ac is removed or reduced to a safe level, however, 227Th will continue to decay to 223Ra with a half-life of just under 19 days. Since 223Ra is an alkaline earth metal it will not easily be coordinated by ligands designed for thorium or other actinides. This 223Ra then forms the beginning of a potentially uncontrolled (untargeted) decay chain including 4 alpha-decays and 2 beta-decays before reaching stable 207Pb. These are illustrated in the table below:
Nuclide227Th223Ra219Rn215Po211Pb211Bi207Tl207Pb½-life18.7 d11.4 d4.0 s1.8 ms36.1 m2.2 m4.8 mstableα-energy/6.155.646.757.396.55MeVβ-energy1.371.42(max)/MeVEnergy %17.516.019.121.03.918.64.0
It is evident from the above two decay tables that 223Ra cannot be entirely eliminated from any preparation of 227Th because the latter will constantly be decaying and generating the former. It is clear, however, that more than 25 MeV in radiated energy will be released from the decay of each 223Ra nucleus administered to a patient, before that nucleus reaches a stable isotope. It is also probable that such 223Ra will not be bound and targeted by the systems of chelation and specific binding designed to transport 227Th to its site of action, due to the differing chemical nature of the two elements. Therefore, for the purpose of targeted cell killing, maximising the therapeutic effect and minimising side-effects, it is important that as much 223Ra as realistically possible should be removed from any 227Th preparation prior to administration.
Separation of 227Th from 223Ra could be carried out quickly and conveniently in a radiological laboratory such as at the site of generation by decay of 227Ac. However, this would not always be possible and may not achieve the desired result effectively because the resulting purified 227Th must then be transported to the site of administration. If this site of use is remote from the site of origin of the 227Th then a further build-up of 223Ra will occur during storage and transport.
In view of the above, it would be a considerable advantage to provide a robust and effective method of purifying 227Th from contaminant 223Ra which could be carried out at a location, such as a centralized location, from which the purified 227Th can reach the site of administration significantly more quickly than the half-life of the isotope. Where the purified isotope will be stored from some time (e.g. 12 to 60 hours) then the method should preferably provide a very high degree of removal of 223Ra so that only radium caused by unavoidable in-growth is administered to the subject without any significant increase due to residual impurity. Alternatively, purification may take place at or close to the point-of-care, at or shortly before the time of administration utilising a simple method that would not require extensive training and experience to carry out. It would be a further advantage if this method could be implemented with a simple group of reagents and items of apparatus, which could be supplied for such a contemporaneous preparation, optionally in the form of a kit. In either embodiment, the method should be robust, reliable and effective, since the resulting purified 227Th may be used directly in pharmaceutical preparation.
Previously known preparations for 227Th have generally been for laboratory use and/or not tested for purity to pharmaceutical standards. In WO2004/091668, for example, 227Th was prepared by anion exchange from a single column and used for experimental purposes without validation of the purity. The primary aim of separation in most preparative methods for 227Th has been the removal of the long-lived 227Ac parent isotope. Methods have not previously been devised or optimised for removal of 223Ra which has grown-in in a 227Th sample previously purified from 227Ac. Furthermore, there are few, if any, documented methods for preparing pharmaceutical standard 227Th that conform to or are suitable for conforming to Good Manufacturing Practice (GMP) principles. It would be an advantage to provide an effective and reliable method that could readily be validated and documented in accordance with GMP working practices.
Brief Description of the Invention
The present inventors have now established that a quick and simple purification procedure may be used to remove 223Ra and its daughter isotopes from a preparation of 227Th using a single strong base anion exchange resin. In this way, a 227Th solution of very high radiochemical purity may be produced while providing a number of desirable advantages in the method.
In a first aspect, the present invention therefore provides a method for the purification of 227Th from a mixture comprising 227Th and 223Ra, said method comprising:                i) preparing a first solution comprising a mixture of 227Th and 223Ra ions dissolved in an aqueous solution of first mineral acid;        ii) loading said first solution onto a strong base anion exchange resin;        iii) eluting 223Ra from said strong base anion exchange resin using a second mineral acid in an aqueous solution;        iv) optionally rinsings said strong base anion exchange resin using a first aqueous medium;        v) eluting 227Th from said strong base anion exchange resin using a third mineral acid in an aqueous solution whereby to generate a second solution comprising 227Th.        
The process will optionally and preferably also include at least one of the following further steps, each generally conducted after steps i) to v) above:                vi) assaying for the 227Th content of said second solution;        vii) evaporating the liquid from said second solution;        viii) forming at least one radiopharmaceutical from at least a portion of the 227Th contained in said second solution;        ix) sterile filtering said radiopharmaceutical.        
In a further aspect, the present invention provides a solution or other sample of 227Th comprising less than 10 KBq, preferably less than 5 KBq (e.g. less than 2 KBq) 223Ra per 1 MBq 227Th. Such a solution is optionally formed or formable by any of the methods herein described, and is preferably formed or formable by the preferred methods herein described. Correspondingly, the methods of the invention are preferably for the formation of a solution of 227Th comprising less than 10 KBq, preferably less than 5 KBq 223Ra per 1 MBq 227Th. A corresponding pharmaceutical preparation is also provided, which may be sterile and may comprise at least one complexing agent (especially for 227Th), at least one targeting agent (e.g. conjugated to said complexing agent), and optionally at least one pharmaceutically acceptable carrier or diluent.
In a still further aspect, the invention also provides a kit (typically a kit for carrying out a method of the invention) comprising a mixture of 227Th and 223Ra, a first mineral acid, a strong base anion exchange resin, a second mineral acid, a first aqueous medium, and a third mineral acid. The kit may further comprise container closures, adapters, syringes, needles, evaporation tubing kit and/or a sterile filter. The mixture of 227Th and 223Ra (as with the first solution in other aspects of the invention) will typically also comprise further 223Ra daughter products. Such a mixture may be the result of radioactive decay of purified or partially purified 227Th during storage and/transportation.